This invention relates to load-tap changers for tapped regulating transformers, and more particularly for load-tap changers of the Jansen type. Such load-tap changers include a selector switch used to select the desired tap on a tapped transformer winding, and a transfer switch used to effect tap changes without interrupting the flow of the load current. Selector switches do not make and break, and transfer switches do make and break, energized circuits.
This invention refers more specifically to polyphase Jansen-type transfer switches as shown, for instance, in U.S. Pat. No. 3,396,254 to A. Bleibtreu, Aug. 6, 1968 for ARRANGEMENT FOR AVOIDING EDDY CURRENT LOSSES IN TRANSFER SWITCH AND SELECTOR SWITCH UNITS WITH INTERPOSED GEAR DRIVE and in U.S. Pat. No. 3,671,687 to A. Bleibtreu, June 20, 1972 for TRANSFER SWITCH FOR TAP-CHANGING REGULATING TRANSFORMER INCLUDING LOST MOTION INTERCONNECTION DRIVING MECHANISM.
The primary object of the present invention is to cope effectively by the provision of non-linear voltage surge protective resistors with voltage surges which may occur in equipment of the kind under consideration. Resistors the resistivity of which decreases inversely to the magnitude of the applied voltage are widely used in the art of surge voltage protection. The present invention relates more specifically to the application of voltage surge protective resistors to transfer switches for tapped transformer windings.
Tapped transformer windings are capable of generating very high voltage surges. The magnitude of such voltage surges depends largely upon the structure of the transformer and upon the nature of winding sections situated between a pair of immediately adjacent taps. Oscillatory build-up phenomena occurring in sections of a transformer winding may tend to damage the winding as well as a transfer switch which is connected to it. Among the most critical voltage surges which may impair a transfer switch are those which occur between a selected current carrying and a pre-selected noncurrent carrying tap of a tapped transformer winding. The aforementioned criticality of such voltage surges results from the fact that they appear on various points of a load-tap changer, at the selector switch, at the bushings of the transfer switch, etc. Increasing the dimensions of a load-tap changer and of its transfer switch so as to be capable of withstanding the voltage surges that may occur therein would result in prohibitive cost, and in prohibitive bulk. For these reasons it is necessary to provide means for limiting the magnitude of voltage surges to which a transfer switch may be subjected, e.g. to 120 kV. This is currently generally achieved by non-linear surge protective resistors which interconnect permanently a selected current carrying tap with a selected noncurrent carrying tap.
In the past the provision of such surge protective resistors resulted in a real danger to the equipment in connection with which they were used. Such resistors undergo changes which may be due to various reasons, including thermal ageing, resulting in instability of the current path formed by the resistors. In the worst possible case the resistance of the voltage surge protective resistor may drop to zero, thus resulting in a short-circuit of the section of the tapped winding situated between two taps which are interconnected by the resistor. Such a short-circuit may cause severe damage, including total destruction of the regulating transformer.
Heretofore non-linear tap-interconnecting voltage surge protective resistors were arranged in the same space as the tapped transformer and the selector switch, which is generally an oil-filled tank. This tends to make matters even worse because it renders the resistors relatively inaccessible for maintenance checks and repairs.
Another means for protecting transfer switches in load-tap changers against surge voltages is the provision of protective spark gaps therein supposed to break down incident to a voltage surge of predetermined magnitude. The provision of spark gaps for the above purpose is, however, subject to very serious limitations and drawbacks. It is difficult to maintain precisely the required spacing between the electrodes of a spark gap, and its breakdown voltage changes also with the dielectric -- normally oil -- in which the spark gap is submersed. The response characteristics of spark gaps are, therefore, wide bands rather than lines. Once a spark gap breaks down, the electric discharge across the gap may continue indefinitely. The fact that the response characteristics of spark gaps are wide bands may result in a gap breakdown during routine voltage tests, and such a gap breakdown cannot be distinguished from an insulation breakdown somewhere in the load-tap changer.
These were the principal reasons for the trend of using non-linear voltage surge protective resistors rather than arc gaps for the protection of load-tap changers. However, as is apparent from the above, the current use of non-linear resistors as surge voltage protectors in load-tap changers is likewise not a satisfactory or safe measure.